With the scream of a circular saw cutting metal as a soundtrack, Tom Brussard led me around to the back of a refrigerated truck that was parked in a vast Medford garage. A team of three workers was installing a new system in the back of the truck that would allow the driver to easily move around a set of plastic curtains that keep in the cold.

As the driver made deliveries of fresh produce, emptying the truck, he’d be able to slide the curtains forward, shrinking the space that needed to be kept cold. The result: “The refrigeration unit on the truck works less, and less run-time not only means the truck uses less diesel fuel, but also that there’s less maintenance on the unit itself,” Brussard says.

Welcome to the new world of energy entrepreneurship in Massachusetts. Entrepreneurs are gravitating to what you might call “low-hanging fruit”: opportunities to improve the way existing systems work, without raising zillions of dollars from venture capitalists or the federal government. And that makes sense, after a short stretch in which three Massachusetts companies — A123 Systems, Beacon Power, and Konarka Technologies — went bankrupt after scarfing up hundreds of millions of dollars from private investors and the Department of Energy. All three set up their own production facilities and bet on major industry shifts, like battery-powered vehicles quickly finding spots in driveways.

“It’s clear that the market is adjusting,” says Radu Gogana, a product development engineer at Yshape, a start-up in Nashua that is developing a new energy monitoring device. “A lot of clean-tech investors have been spooked by the big failures, and so a lot of the huge, crazy projects just aren’t getting funded anymore.”

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Yshape is trying to raise just $60,000 on the fund-raising site Kickstarter to start producing a new device called the Heat Meter. It gets affixed magnetically to your furnace, and it senses the vibrations when the furnace is operating. When you tell the Heat Meter website about your home’s size and location, and the kind of heating system you have, it can then provide information about how your energy consumption compares with that of similar homes.

“We’re able to provide home­owners with information that is sort of like the m.p.g. for their home, allowing them to know where they stand,” says Gogana, who received his master’s in mechanical engineering from MIT last year. “A lot of people don’t know if they live in a Prius or a Hummer when it comes to energy efficiency. We can also point them to really cheap improvements that have a fast payback, like adding insulation and sealing cracks.”

The Heat Meter device sells for $150, and the company expects to start shipping them to customers this summer.

Brussard relied on a Bank of America credit line to develop and patent his SmartSlider system for refrigerated trucks. It sells for $2,000 and has been installed in more than 40 trucks so far, he says. He built the prototype in his backyard.

But Brussard admits that figuring out how to do large-scale marketing on a shoestring budget is a challenge. The same is true at Energy Efficiency Solutions, a Holliston company that sells the Burner Booster.

Inventor Eric LaVoie started working on the product in 2006, in his garage. But the company has sold just 40 units, after raising about $2 million from individual investors. The $5,000 product is a high-pressure fuel injection system. By spraying the oil into a heating system’s burner at a higher pressure, it creates a mist of fuel droplets that burns more completely.

LaVoie’s company is primarily focused on businesses and institutions, which use much more oil than homes. (One system is in use at the state prison in Plymouth, where it heats water and a dormitory building.)

“People sometimes say that the technology sounds too good to be true, so we guarantee that they will save at least 15 percent on fuel, or we refund the purchase,” LaVoie says. In reality, fuel savings typically exceed 20 percent, he says. Still, “it’s going slower than we expected.” (One issue is the appeal of converting to less-expensive natural gas, though LaVoie is trying to enlist fuel oil distributors to help connect with customers who don’t have that option.)

It used to be assumed that most new energy businesses would require vast amounts of money before the wind turbine blades would start spinning. But Gogana at Yshape says he can’t think of anyone locally who’s working on an ambitious new energy technology right now and “trying to raise a couple hundred million for it.”

Udi Meirav’s last start-up, Luminus Devices, raised more than $120 million and built two production facilities to crank out a new kind of light-emitting diode. Now, he’s developing a new approach to making industrial air conditioning systems more efficient — and, so far, he’s doing it with about $1 million.

With eight employees in Massachusetts and Israel, his start-up, EnVerid, is working on a system that can be attached to existing air conditioners to pull carbon dioxide and volatile organic compounds from the air. That lets the air conditioner recirculate air that has already been cooled, rather than venting it out and pulling in fresh, warm air to then cool. The goal is to cut energy use by at least 20 percent. EnVerid has done some early testing in Israel and plans to continue that in Texas this spring.

As for today’s scrappier, “capital efficient” approach to developing technologies that curb energy consumption, Meirav says, “The jury is still out as to whether you can turn a company into a big success this way. But there’s not much of a choice right now.”

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